Clean's method for dealing with mutable state and I/O is done through a uniqueness typing system, in contrast to Haskell's use of monads. The compiler takes advantage of the uniqueness type system to generate more efficient code, because it knows that anything with a uniqueness type can only be used once. Therefore, a unique value can be changed in place.[2]

The type declaration states that the function is a right associative infix operator with priority 8: this states that x*x^(n-1) is equivalent to x*(x^(n-1)) as opposed to (x*x)^(n-1). This operator is pre-defined in StdEnv, the Clean standard library.

Computation is based on graph rewriting and reduction. Constants such as numbers are graphs and functions are graph rewriting formulas. This, combined with compilation to native code, makes Clean programs run relatively fast, even with high abstraction.[3]

To close the gap between Core Clean, a high-level functional language, and machine code, the ABC-machine is used. This is an imperative abstract graph rewriting machine.[4] Generating concrete machine code from abstract ABC code is a relatively small step, so by using the ABC-machine it is much easier to target multiple architectures for code generation.

The ABC-machine has a uncommon memory model. It has a graph store to hold the Clean graph that is being rewritten. The A(rgument)-stack holds arguments that refer to nodes in the graph store. This way, a node's arguments can be rewritten, which is needed for pattern matching. The B(asic value)-stack holds basic values (integers, characters, reals, etc.). While not strictly necessary (all these elements could be nodes in the graph store as well), using a separate stack is much more efficient. The C(ontrol)-stack holds return addresses for flow control.

The runtime system, which is linked into every executable, has a print rule which prints a node to the output channel. When a program is executed, the Start node is printed. For this, it has to be rewritten to root normal form, after which its children are rewritten to root normal form, etc., until the whole node is printed.

Clean is dual licensed: it is available under the terms of the GNU LGPL, and also under a proprietary license. For the libraries, runtime system and examples, not the GNU LGPL but the Simplified BSD License applies.

A benchmark from 2008 shows that Clean is faster than Haskell in most cases:[5]

Speed comparison of five compilers (time in seconds)

Language

Pri

Sym

Inter

Fib

Match

Ham

Twi

Qns

Kns

Parse

Plog

Qsort

Isort

Msort

SAPL Int

6.1

17.6

7.8

7.3

8.5

15.7

7.9

6.5

47.1

4.4

4.0

16.4

9.4

4.4

SAPL Bas

4.3

13.2

6.0

6.5

5.9

9.8

5.6

5.1

38.3

3.8

2.6

10.1

6.7

2.6

GHC

2.0

1.7

8.2

4.0

4.1

8.4

6.6

3.7

17.7

2.8

0.7

4.4

2.3

3.2

GHC -O

0.9

1.5

1.8

0.2

1.0

4.0

0.1

0.4

5.7

1.9

0.4

3.2

1.9

1.0

Clean

0.9

0.8

0.8

0.2

1.4

2.4

2.4

0.4

3.0

4.5

0.4

1.6

1.0

0.6

As can be seen, Clean outruns Haskell (GHC) on almost all test cases. Only parser combinators are faster in Haskell. Using GHC -O we get some optimisations, making pattern matching and higher order functions faster than in Clean as well. In most cases, however, Clean outperforms GHC -O or at least isn't slower.